1. Field of the Invention
This invention is related to a method for growth of semipolar (Al, In, Ga, B)N optoelectronic devices.
2. Description of the Related Art
(Note: This application references a number of different publications and patents as indicated throughout the specification by one or more reference numbers within brackets, e.g., [x]. A list of these different publications and patents ordered according to these reference numbers can be found below in the section entitled “References.” Each of these publications and patents is incorporated by reference herein.)
The usefulness of gallium nitride (GaN), and its ternary and quaternary compounds incorporating aluminum and/or indium (AlGaN, InGaN, and AlInGaN), has been well established for fabrication of visible and ultraviolet (UV) optoelectronic devices and high-power electronic devices. These devices are typically grown epitaxially using growth techniques including metalorganic chemical vapor deposition (MOCVD), molecular beam epitaxy (MBE), and hydride vapor phase epitaxy (HVPE).
GaN and its alloys are most stable in the hexagonal würtzite crystal structure, described by two (or three) equivalent basal plane axes that are rotated 120° with respect to each other (the a-axes), all of which are perpendicular to a unique c-axis. Group III atoms and nitrogen atoms occupy alternating c-planes along the crystal's c-axis. The symmetry elements included in the würtzite structure dictate that III-nitrides possess a bulk spontaneous polarization along this c-axis, and the würtzite structure exhibits piezoelectric polarization.
Current nitride technology for electronic and optoelectronic devices employs nitride films grown along the polar c-direction. However, conventional c-plane quantum well structures in III-nitride based optoelectronic and electronic devices suffer from the undesirable quantum-confined Stark effect (QCSE), due to the existence of strong piezoelectric and spontaneous polarizations. The strong built-in electric fields along the c-direction cause spatial separation of electrons and holes that in turn give rise to restricted carrier recombination efficiency, reduced oscillator strength, and red-shifted emission.
One approach to eliminating the spontaneous and piezoelectric polarization effects in GaN optoelectronic devices is to grow the devices on nonpolar planes of the crystal. Such planes contain equal numbers of gallium (Ga) and nitrogen (N) atoms and are charge-neutral. Furthermore, subsequent nonpolar layers are crystallographically equivalent to one another, so the crystal will not be polarized along the growth direction. Two such families of symmetry-equivalent nonpolar planes in GaN are the {11 20} family, known collectively as a-planes, and the {1 100} family, known collectively as m-planes. Unfortunately, despite advances made by researchers at the University of California, Santa Barbara, growth of nonpolar nitrides remains challenging and has not yet been widely adopted in the III-nitride industry.
Another approach to reducing, or possibly eliminating, the polarization effects in GaN optoelectronic devices is to grow the devices on semipolar planes of the crystal. The term semipolar planes can be used to refer to a wide variety of planes that possess two nonzero h, k, and thus i, Miller indices, and a nonzero 1 Miller index. Some commonly observed examples of semipolar planes in c-plane GaN heteroepitaxy include the {11 22}, {10 11}, and {10 13} planes, which are found in the facets of pits. These planes also happen to be the same planes researchers at the University of California, Santa Barbara, have grown in the form of planar films. Other examples of semipolar planes in the würtzite crystal structure include, but are not limited to, {10 12}, {20 21}, and {10 14}. The nitride crystal's polarization vector lies neither within such planes nor normal to such planes, but rather lies at some angle inclined relative to the plane's surface normal. For example, the {11 22} and {10 13} planes are at 58.43° and 32.06° to the c-plane, respectively. The inter-planar angles between a given semipolar plane and the c-plane are listed in Table 1 below.
TABLE 1Angles of semipolar planes relative to the c-planeSemipolar Plane10-1210-1410-1311-2410-1211-2311-2210-1111-2110-1-2Inclined18.76°25.16°32.06°39.14°43.22°47.33°58.43°61.98°72.92°74.80°Angle toc-plane
In addition to spontaneous polarization, the second form of polarization present in nitrides is piezoelectric polarization. This occurs when the material experiences a compressive or tensile strain, as can occur when (Al, In, Ga, B)N layers of dissimilar composition (and therefore different lattice constants) are grown in a nitride heterostructure. For example, a thin AlGaN layer on a GaN template will have in-plane tensile strain, and a thin InGaN layer on a GaN template will have in-plane compressive strain, both due to lattice matching to the GaN. Therefore, for an InGaN quantum well on GaN, the piezoelectric polarization will point in the opposite direction than that of the spontaneous polarization of the InGaN and GaN. For an AlGaN layer latticed matched to GaN, the piezoelectric polarization will point in the same direction as that of the spontaneous polarization of the AlGaN and GaN.
The advantage of using semipolar planes over c-plane nitrides is that the total polarization will be reduced. There may even be zero polarization for specific alloy compositions on specific planes. Such scenarios are discussed in detail in these scientific papers [4,5]. The important point is that the polarization will be reduced compared to that of c-plane nitride structures.
Semipolar GaN planes have been demonstrated on the sidewalls of patterned c-plane oriented stripes. Nishizuka et al. [1] have grown {11 22} InGaN quantum wells using this technique. They have also demonstrated that the internal quantum efficiency of the semipolar plane {11 22} is higher than that of the c-plane, which results from the reduced polarization. Nishizuka et al.'s method involves patterning stripes of a mask material, often SiO2 for GaN. The GaN is grown from open windows between the mask and then grown over the mask. To form a continuous film, the GaN is coalesced by lateral growth. The facets of these stripes can be controlled by the growth parameters. If the growth is stopped before the stripes coalesce, then a small area of semipolar plane can be exposed. The semipolar plane will have a certain degree of inclination to the substrate surface. For example, Nishizuka et al. grew a GaN plane at a 58.43° angle with respect to the c plane, and InGaN quantum wells on top of the GaN plane [2].
Although the Nishizuka method provided high crystal quality semipolar orientation facets, it resulted in the formation of multiple crystal facets, including large areas with a polar {0001} or non-polar {11 20} orientation. More importantly, the area of the semipolar orientation facet is rather small and non-continuous, and therefore growing and fabricating electrically injected optoelectronic devices on those semipolar orientation facets becomes almost impossible.
Our research group at the University of California, Santa Barbara, has been engaged in an effort to produce planar semi-polar nitride films with a large area of (Al, In, Ga, B)N parallel to the substrate surface, and suitable for use in electrically injected optoelectronic devices. Our research group has successfully grown semipolar InGaN/GaN multiple quantum well (MQW) light emitting diodes (LEDs) via MOCVD using an HYPE-grown 20 μm thick {10 13} GaN template heteroepitaxially deposited on an m-plane sapphire substrate [3]. Our semipolar LEDs show a limited blue-shift of the electroluminescence peak wavelength with increasing drive current, as compared to c-plane nitride devices, signaling a dramatic reduction of the QCSE. Reduction of the QCSE theoretically leads to a greater internal quantum efficiency; nevertheless, the output power of current semipolar orientation LEDs is rather low, i.e., at a 20 mA driving current, the output power is 120 μW, and the maximum output power is only 250 μW. For comparison, c-plane nitride LEDs have milliwatt (mW) range output power.
The lower semipolar nitride LED output power can be attributed to the high density of non-radiative recombination centers resulting from the rather poor crystal quality. Since the growth of nitride semiconductor is heteroepitaxial, the large difference between the lattice constants of nitride semiconductor layers and the substrate material generates many dislocations, which act as non-radiative recombination centers. In order to achieve better quality crystal growth of semipolar nitride semiconductor layers, further optimization of growth conditions, methodologies, and technology is required.
In summary, growing and fabricating high-quality semipolar orientation electrically injected nitride optoelectronic devices (also referred to as semipolar devices) using current technologies is rather difficult, and it will take a lot of research effort to make those devices comparable in performance to their c-orientation counterparts. The purpose of the present invention described herein is to produce high-quality semipolar devices homoepitaxially, through the use of current sophisticated c-orientation (Al, Ga, In, or B) nitride growth, etching, and epitaxial lateral overgrowth (ELO) techniques.